Bug 257370

WPT: https://wpt.fyi/results/css/css-counter-styles/counter-style-at-rule/disclosure-styles.html?label=master&label=experimental&aligned&q=safari%3Afail%20

Spec: https://drafts.csswg.org/css-counter-styles/#simple-symbolic

> For the disclosure-open and disclosure-closed counter styles, the marker must be an image or character suitable for indicating the open and closed states of a disclosure widget, such as HTML’s details element. If the image is directional, it must respond to the writing mode of the element, similar to the bidi-sensitive images feature of the Images 4 module. For example, the disclosure-closed style might use the characters U+25B8 BLACK RIGHT-POINTING SMALL TRIANGLE (▸) and U+25C2 BLACK LEFT-POINTING SMALL TRIANGLE (◂), while the disclosure-open style might use the character U+25BE BLACK DOWN-POINTING SMALL TRIANGLE (▾).

This is partially implemented in WebKit in RenderListMarker, but it doesn't apply to counter styles that extend the built-in disclosure styles.